feat: 添加 Dashboard 供应商成本统计功能

- 新增 stats_daily_provider 表存储每日供应商统计数据
- 实现供应商维度的数据聚合服务
- Dashboard API 返回 provider_summary 供应商汇总数据
- 前端新增 DoughnutChart 环形图组件
- Dashboard 新增供应商成本分布可视化卡片
- 移除重复的请求次数/费用趋势折线图

Closes #110

Co-authored-by: RWDai <27391645+RWDai@users.noreply.github.com>

frontend/src/components/charts/DoughnutChart.vue

@@ -0,0 +1,113 @@
+<template>
+  <div class="w-full h-full">
+    <canvas ref="chartRef" />
+  </div>
+</template>
+
+<script setup lang="ts">
+import { ref, onMounted, onUnmounted, watch, nextTick } from 'vue'
+import {
+  Chart as ChartJS,
+  ArcElement,
+  DoughnutController,
+  Title,
+  Tooltip,
+  Legend,
+  type ChartData,
+  type ChartOptions
+} from 'chart.js'
+
+ChartJS.register(
+  ArcElement,
+  DoughnutController,
+  Title,
+  Tooltip,
+  Legend
+)
+
+interface Props {
+  data: ChartData<'doughnut'>
+  options?: ChartOptions<'doughnut'>
+  height?: number
+}
+
+const props = withDefaults(defineProps<Props>(), {
+  height: 300,
+  options: undefined
+})
+
+const chartRef = ref<HTMLCanvasElement>()
+let chart: ChartJS<'doughnut'> | null = null
+
+const defaultOptions: ChartOptions<'doughnut'> = {
+  responsive: true,
+  maintainAspectRatio: false,
+  cutout: '60%',
+  plugins: {
+    legend: {
+      position: 'right',
+      labels: {
+        color: 'rgb(107, 114, 128)',
+        usePointStyle: true,
+        padding: 16,
+        font: { size: 11 }
+      }
+    },
+    tooltip: {
+      backgroundColor: 'rgb(31, 41, 55)',
+      titleColor: 'rgb(243, 244, 246)',
+      bodyColor: 'rgb(243, 244, 246)',
+      borderColor: 'rgb(75, 85, 99)',
+      borderWidth: 1,
+      callbacks: {
+        label: (context) => {
+          const value = context.raw as number
+          const total = (context.dataset.data as number[]).reduce((a, b) => a + b, 0)
+          const percentage = total > 0 ? ((value / total) * 100).toFixed(1) : '0'
+          return `${context.label}: $${value.toFixed(4)} (${percentage}%)`
+        }
+      }
+    }
+  }
+}
+
+function createChart() {
+  if (!chartRef.value) return
+
+  chart = new ChartJS(chartRef.value, {
+    type: 'doughnut',
+    data: props.data,
+    options: {
+      ...defaultOptions,
+      ...props.options
+    }
+  })
+}
+
+function updateChart() {
+  if (chart) {
+    chart.data = props.data
+    chart.update('none')
+  }
+}
+
+onMounted(async () => {
+  await nextTick()
+  createChart()
+})
+
+onUnmounted(() => {
+  if (chart) {
+    chart.destroy()
+    chart = null
+  }
+})
+
+watch(() => props.data, updateChart, { deep: true })
+watch(() => props.options, () => {
+  if (chart) {
+    chart.options = { ...defaultOptions, ...props.options }
+    chart.update()
+  }
+}, { deep: true })
+</script>